1. Field of the Invention
The present invention relates to a position measurement device, and in particular to an encoder that includes an alignment target having a designed in offset between periodic patterns.
2. Discussion of the Related Art
Position measurement devices, such as optical encoders, are well known in the art. FIG. 1 shows a conventional optical encoder 10 that includes a light source 12, which produces a light beam 13. The light beam 13 passes through a grid plate 14 that includes a number of regular opaque and transparent regions 16. After passing through the grid plate 14, the light beam 13 is received by a detector 18. As the grid plate 14 moves relative to the light source 12 and detector 18, as indicated by the arrows in FIG. 1, the light beam 13 will pass through only the transparent regions 16 on grid plate, creating pulses of light at the detector 18. The detector 18 converts the pulses of light into a periodic electronic signal. By counting the pulses from the detector 18, and knowing the length of each transparent and opaque region, the position of the grid plate 14 relative to the light source 12 and detector 18 can be determined.
Encoder 10 is a relatively inexpensive device, but unfortunately, the accuracy of the device is limited. Techniques such as interpolation may be used to improve the accuracy, but even with interpolation the accuracy is limited to approximately 20 nm. Another type of encoder that is used to provide an accurate position measurement is a differential interferometer.
FIG. 2 shows a conventional differential interferometer alignment system 50 that may be used as an encoder. Alignment system 50 includes a light source 52 that produces monochromatic light. Typically, the monochromatic light is collimated. A beam splitter 54 splits the light to be reflected off a stationary mirror 56 and a mirror 58, which is movable, as indicated by the arrows in FIG. 2. Mirror 58 is typically mounted on a movable object, e.g., a stage. The light is reflected off mirrors 56 and 58 and is recombined at beam splitter 54. The combined light is focused by a lens 60 onto a detector 62. The operation of a differential interferometer is well known. Advantageously, alignment system 50 provides an accuracy of approximately 10 nm, but typically has increased cost relative to a simpler encoder, such as that shown in FIG. 1.
Thus, there is a need for an inexpensive but highly accurate encoder.